1. Field of the Invention
The present invention relates to a field effect transistor for use in high-speed logic integrated circuits, etc.
2. Description of the Prior Art
As a field effect transistor (FET) which is used as a device for high frequency amplifiers, high-speed computers, etc., a schottky qatc FET (MESFET) using, as its schottky gate, GaAs which has mobility in a low electric field higher than Si (GaAs MESFET), a high electron mobility transistor (HEMT), etc. have vigorously been developed recently.
In order to enhance the performance of a basic device for an integrated circuit for a high-speed computer, particularly the integration scale and the operation velocity, it is very important to realize high current driving-ability. For this, it is necessary to increase the velocity of a carrier and the carrier density, as it can easily be understood from a most simple model wherein the electric current flowing through a channel is in proportion to the product of the carrier density and the carrier velocity. It is also necessary to make the channel layer thinner, for suppressing short channel effects caused by shortening the gate length.
For these reasons, heretofore, for example in the case of said GaAs MESFET, a molecular-beam opitaxy (MBE) method, which is usable for depositing a thin film with the best controllability, has been used for growing the thin film in order to increase the carrier density, and make the channel thinner. The limit of doping with practical controllability by said MBE method, however, is about 2.times.10.sup.18 cm.sup.-3.
GaAs has a large electron mobility and so is advantageous for use as a high-speed device material, but owing to its low density state, its doping concentration is limited. On the other hand, Si has a lower electron mobility than GaAs, but has a higher density state sufficient for realizing a high density, not lower than 10.sup.20 cm.sup.-3. Thus the density state and the mobility of a carrier have a reverse correlation determined by an intrinsic band spectrum construction of the material. This means that in principle it is impossible to increase both of the density state and the carrier mobility.